grilo: Fix warnings when clicking on an item

When the search is on. The search results might not be the ones
displayed. So make sure that we dismiss the search bar, and
activate the clicked item.
grilo: Don't warn when setting invalid search source

Instead, return a boolean to show failure. This allows us
to try to set the search easily without having to find
whether it supports search.
grilo: Don't warn if the media is gone for a thumbnail

When retrieving thumbnails, it's possible that the item
is already out of view. So don't throw errors in that case.
grilo: Automatically set the source when starting a search

When browsing within a source, starting a search should search
in that source by default.
main: Remove transitions between player/browser
main: Fix OSD toolbar button not being square